Ok question one, if I try to make a mold by painting silicone onto clay will I need to seal the clay first? If so with what?

How do I control the thickness of the final cast?

What's a good silicone mold starter kit that I can buy off amazon? And since I want to do a few designs, how should I buy replacement silicone? Are there any types of silicone I should avoid? Do I care about getting any on the floor, or will it just peel up?

Finally, are there any videos around that'll get me ready to try molding clear lenses? I'm pretty sure that's beyond me at this point but I want to go there eventually.